Search Results (36 total)

A conceptual molecular phase-coherent transistor is proposed based on first-principles quantum transport calculations. The device is formed by two molecules connected via a one-dimensional wire and it is operated by gating the interconnect between the molecules and exploiting quantum-mechanical interference. The transistor thus works by controlling the electron phase instead of the position of the molecular energy levels, and it paves the way for phase-controllable electronic circuits.

The linear term of the near-nucleus expansion of the spherically averaged exchange-correlation potential υ̅_xc(r) in density functional theory (DFT) is shown to be nonzero and to arise solely from the correlation-kinetic effects. Analytical expressions for it and for those of the separate exchange υ_x(r) and correlation υ_c(r) potentials are derived. The results were also obtained recently via quantal DFT, but here are obtained via ordinary Hohenberg-Kohn-Sham DFT. It is further pointed out that the linear term in υ_xc(r) arising mainly from υ_c(r) is rather small, and υ_xc(r) therefore has a nearly quadratic structure near the nucleus. Implications of the results for the construction of the Kohn-Sham system are discussed and examples are given.

In local effective potential theories of electronic structure, the electron correlations due to the Pauli exclusion principle, Coulomb repulsion, and correlation-kinetic effects, are all incorporated in the local electron-interaction potential v_ee(r). In previous work, it has been shown that for spherically symmetric or sphericalized systems, the asymptotic near-nucleus expansion of this potential is v_ee(r)=v_ee(0)+βr+O(r²), with v_ee(0) being finite. By assuming that the Schrödinger and local effective potential theory wave functions are analytic near the nucleus of atoms, we prove the following via quantal density functional theory (QDFT): (i) Correlations due to the Pauli principle and Coulomb correlations do not contribute to the linear structure; (ii) these Pauli and Coulomb correlations contribute quadratically; (iii) the linear structure is solely due to correlation-kinetic effects, the contributions of these effects being determined analytically. We also derive by application of adiabatic coupling constant perturbation theory via QDFT (iv) the asymptotic near-nucleus expansion of the Hohenberg-Kohn-Sham theory exchange v_x(r) and correlation v_c(r) potentials. These functions also approach the nucleus linearly with the linear term of v_x(r) being solely due to the lowest-order correlation kinetic effects, and the linear term of v_c(r) being due solely to the higher-order correlation kinetic contributions. The above conclusions are equally valid for systems of arbitrary symmetry, provided spherical averages of the properties are employed.

Inelastic lifetime of an electron quasiparticle in an electron liquid due to electron-electron interaction evaluated in previous work is calculated in an alternative way. Both the contributions of the “direct” and “exchange” processes are included. The results turn out to be exactly the same as those obtained previously, and hence confirm the latter and consequently fully resolve the theoretical discrepancies existing in the literature. Derivation in the two-dimensional case is presented in detail due to its intricacies. The effects of local field and finite well width on the effective electron interaction in the two-dimensional case are also investigated in a quantitive comparison of the electron relaxation rate between theory and experiment. These effects are shown to make a rather small contribution to the quasiparticle lifetime.

We describe a model of damage in rf cavities and show how this damage can limit cavity operation. We first present a review of mechanisms that may or may not affect the ultimate fields that can be obtained in rf cavities, assuming that mechanical stress explains the triggers of rf breakdown events. We present a method of quantifying the surface damage caused by breakdown events in terms of the spectrum of field enhancement factors, β, for asperities on the surface. We then model an equilibrium that can develop between damage and conditioning effects, and show how this equilibrium can determine cavity performance and show experimental evidence for this mechanism. We define three functions that quantify damage, and explain how the parameters that determine performance can be factored out and measured. We then show how this model can quantitatively explain the dependence of cavity performance on material, frequency, pulse length, gas, power supply, and other factors. The examples given in this paper are derived from a variety of incomplete data sets, so we outline an experimental program that should improve these predictions, provide mechanisms for comparing data from different facilities, and fill in many gaps in the existing data.

The ladder theory, in which the Bethe-Goldstone equation for the effective potential between two scattering particles plays a central role, is well known for its satisfactory description of the short-range correlations in the homogeneous electron liquid. By solving exactly the Bethe-Goldstone equation in the limit of large transfer momentum between two scattering particles, we obtain accurate results for the on-top pair-correlation function g(0), in both three dimensions and two dimensions. Furthermore, we prove, in general, that the ladder theory satisfies the cusp condition for the pair-correlation function g(r) at zero distance r=0.

The exchange-correlation kernel in the spin channel in an electron liquid has the structure f_xc,−^L,T(q,ω)→q→0A(ω)∕q²+B^L,T(ω) in the limit of the long wavelength. Here L denotes the longitudinal component and T the transverse component relative to the direction of the wave vector q. A collection of exact results for A(ω) and B^L,T(ω) is obtained at limiting low and high frequency, respectively, in two dimensions. Based on these results, we further give approximate interpolations for A(ω) and B^L,T(ω) at all frequencies in the paramagnetic case.

We have measured the effects of high (0–4.5 T) magnetic fields on the operating conditions of 805 MHz accelerating cavities, and discovered that the maximum accelerating gradient drops as a function of the axial magnetic field. While the maximum gradient of any cavity is governed by a number of factors including conditioning, surface topology and materials, we argue that J×B forces within the emitters are the mechanism for enhanced breakdown in magnetic fields. The pattern of emitters changes over time and we show an example of a bright emitter which disappears during a breakdown event. We also present unique measurements of the distribution of enhancement factors, β, of secondary emitters produced in breakdown events during conditioning. We believe these secondary emitters can also be breakdown triggers, and the secondary emitter spectrum helps to determine the maximum operating field.

We calculate the inelastic lifetime of an electron quasiparticle due to Coulomb interactions in an electron liquid at low (or zero) temperature in two and three spatial dimensions. The contribution of “exchange” processes is calculated analytically and is shown to be non-negligible even in the high-density limit in two dimensions. Exchange effects must therefore be taken into account in a quantitative comparison between theory and experiment. The derivation in the two-dimensional case is presented in detail in order to clarify the origin of the disagreements that exist among the results of previous calculations, even the ones that only took into account “direct” processes.

The high-frequency limits of the singular component A(ω) of the small wave-vector expansion of the longitudinal (L) and transverse (T) components of the spin-resolved exchange-correlation kernel tensor fxc,σσ′L,T(q,ω)=−v(q)Gσσ′L,T(q,ω) in a two-dimensional isotropic electron liquid with arbitrary spin polarization are studied. Here Gσσ′L,T(q,ω) is the spin-resolved local-field factor, v(q) is the Coulomb interaction in momentum space, and σ denotes spin. Particularly, the real part of A(ω) is found to be logarithmically divergent at large ω. The large wave-vector structure of the corresponding spin-resolved static structure factor is also established.

We show that in order to calculate correctly the spin current carried by a quasiparticle in an electron liquid one must use an effective “spin mass” m_s that is larger than both the band mass m_b, which determines the charge current, and the quasiparticle effective mass m^*, which determines the heat capacity. We present two independent estimates of the spin mass enhancement, m_s/m_b, in two- and three-dimensional electron liquids, based on (i) previously calculated values of the Landau parameters and (ii) a recent theory of the dynamical local field factor in the spin channel. Both methods yield a significant spin mass enhancement, which is larger in two dimensions than in three.

The components of the exchange-correlation kernel tensor of an isotropic electron liquid in the spin channel have the structure f_xc,-^L,T(q,ω)→q→0A(ω)/q²+B^L,T(ω), where L denotes the longitudinal component and T the transverse component relative to the direction of the wave vector q. In this paper we calculate analytically the high- and low-frequency limits of A(ω) and B^L,T(ω) and combine these limiting forms with the Kramers-Krönig dispersion relations to obtain approximations for A(ω) and B^L,T(ω) at all frequencies.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We present measurements of dark currents and x rays in a six cell 805 MHz cavity, taken as part of an rf development program for muon cooling, which requires high power, high stored energy, low frequency cavities operating in a strong magnetic field. We have done the first systematic study of the behavior of high power rf in a strong (2.5–4 T) magnetic field. Our measurements extend over a very large dynamic range in current and provide good fits to the Fowler-Nordheim field emission model assuming mechanical structures produce field enhancements at the surface. The locally enhanced field intensities we derive at the tips of these emitters are very large, (∼10  GV/m), and should produce tensile stresses comparable to the tensile strength of the copper cavity walls and should be capable of causing breakdown events. We also compare our data with estimates of tensile stresses from a variety of accelerating structures. Preliminary studies of the internal surface of the cavity and window are presented, which show splashes of copper with many sharp cone shaped protrusions and wires which can explain the experimentally measured field enhancements. We discuss a “cold copper” breakdown mechanism and briefly review alternatives. We also discuss a number of effects due to the 2.5 T solenoidal fields on the cavity such as altered field emission due to mechanical deformation of emitters, and dark current ring beams, which are produced from the irises by E×B drifts during the nonrelativistic part of the acceleration process.

It has been known for some time that the exchange-correlation potential in time-dependent density-functional theory is an intrinsically nonlocal functional of the density as soon as one goes beyond the adiabatic approximation. In this paper we show that a much more severe nonlocality problem, with a completely different physical origin, plagues the exchange-correlation potentials in time-dependent spin-density functional theory. We show how the use of the spin current density as a basic variable solves this problem, and we provide an explicit local expression for the exchange-correlation fields as functionals of the spin currents.

We present a quantum-mechanical image-potential theory by determining analytically the Kohn-Sham (KS) exchange-correlation potential v_xc(z) in the classically forbidden region of the metal-vacuum interface. The asymptotic structure of the image potential is determined to be -(α_KS,x+1/4)/z, where α_KS,x depends upon the Fermi energy and barrier height of the metal. The structure is obtained from exact expressions derived for v_xc(r) in the asymptotic region in terms of the electron self-energy. The KS exchange potential is determined as v_x(z)∼-α_KS,x/z, thereby confirming previous work. The correlation part of the self-energy employed is that of the plasmon-pole approximation, and leads to the KS correlation potential v_c(z)∼-1/(4z). The quantum image potential derived therefore, differs from the commonly accepted classical form of -1/(4z). The import of this result to both the theory of image states and the density-functional theory is also discussed.

The imaginary parts of the exchange-correlation kernels f_xc^L,T(ω) in the longitudinal and transverse current-current response functions of a homogeneous electron liquid are calculated exactly at low frequency, to leading order in the Coulomb interaction. Combining these new results with the previously known high-frequency behaviors of Im f_xc^L,T(ω) and with the compressibility and the third moment sum rules, we construct simple interpolation formulas for Im f_xc^L,T(ω) in three and two spatial dimensions. A feature of our interpolation formulas is that they explicitly take into account the two-plasmon component of the excitation spectrum: our longitudinal spectrum Im f_xc^L(ω) is thus intermediate between the Gross-Kohn interpolation, which ignores the two-plasmon contribution, and a recent approximate calculation by Nifosì, Conti, and Tosi, which probably overestimates it. Numerical results for both the real and imaginary parts of the exchange-correlation kernels at typical electron densities are presented, and compared with those obtained from previous approximations. We also find an exact relation between Im f_xc^L(ω) and Im f_xc^T(ω) at small ω.

We derive the spin-wave dynamics of a magnetic material from the time-dependent spin-density-functional theory in the linear response regime. The equation of motion for the magnetization includes, besides the static spin stiffness, a “Berry curvature” correction and a damping term. A gradient expansion scheme based on the homogeneous spin-polarized electron gas is proposed for the latter two quantities, and the first few coefficients of the expansion are calculated to second order in the Coulomb interaction.

Time-dependent quantal density-functional theory (Q-DFT) is a description of the s-system of noninteracting fermions with electronic density equivalent to that of Schrödinger theory, in terms of fields whose sources are quantum-mechanical expectations of Hermitian operators. The theory delineates and defines the contribution of each type of electron correlation to the local electron-interaction potential ν_ee(r,t) of the s system. These correlations are due to the Pauli exclusion principle, Coulomb repulsion, correlation-kinetic, and correlation-current-density effects, the latter two resulting, respectively, from the difference in kinetic energy and current density between the interacting Schrödinger and noninteracting systems. We employ Q-DFT to prove the following sum rules and properties of the s system: (i) the components of the potential due to these correlations separately exert no net force on the system; (ii) the torque of the potential is finite and due solely to correlation-current-density effects; (iii) two sum rules involving the curl of the dynamic electron-interaction kernel defined as the functional derivative of ν_ee(r,t) are derived and shown to depend on the frequency dependent correlation-current-density effect. Furthermore, via adiabatic coupling constant (λ) perturbation theory, we prove: (iv) the exchange potential ν_x(r,t) is the work done in a conservative field representative of Pauli correlations and lowest-order O(λ) correlation-kinetic and correlation-current-density effects; (v) the correlation potential ν_c(r,t) commences in O(λ²), and, at each order, it is the work done in a conservative field representative of Coulomb correlations and correlation-kinetic and correlation-current-density effects; (vi) we derive the integral virial theorem relating ν_ee(r,t) to the electron-interaction and correlation-kinetic energy for arbitrary coupling constant strength λ, and show there are no explicit correlation-current-density contributions to the energy. From this integral virial theorem we (vii) obtain the fully interacting (λ=1) and exchange-only (λ=0) integral virial theorems as special cases, the latter showing there is no explicit correlation-kinetic contribution to the exchange energy; and (viii) write expressions for the electron-interaction and correlation-kinetic actions for arbitrary coupling constant λ in terms of the corresponding fields.

The local electron-interaction potential of Kohn-Sham density functional theory is representative of electron correlations due to the Pauli exclusion principle, Coulomb repulsion, and correlation-kinetic effects which are a consequence of the correlation contribution to the kinetic energy. This potential exhibits a discontinuity as the electron number passes through an integer value. We show, via quantal density functional theory, that the physical origin of the discontinuity is the correlation-kinetic effect. The magnitude of the discontinuity is derived to be the work done to move an electron in a conservative field representative of this effect. An explanation is given as to how these correlation-kinetic effects give rise to such a discontinuity. Further, how this understanding relates to that within traditional Kohn-Sham theory and other previous explanations of the discontinuity is also discussed.

The status of the research on muon colliders is discussed and plans are outlined for future theoretical and experimental studies. Besides work on the parameters of a 3–4 and 0.5 TeV center-of-mass (COM) energy collider, many studies are now concentrating on a machine near 0.1 TeV (COM) that could be a factory for the s-channel production of Higgs particles. We discuss the research on the various components in such muon colliders, starting from the proton accelerator needed to generate pions from a heavy-Z target and proceeding through the phase rotation and decay (π→μν_μ) channel, muon cooling, acceleration, storage in a collider ring, and the collider detector. We also present theoretical and experimental R&D plans for the next several years that should lead to a better understanding of the design and feasibility issues for all of the components. This report is an update of the progress on the research and development since the feasibility study of muon colliders presented at the Snowmass '96 Workshop [R. B. Palmer, A. Sessler, and A. Tollestrup, Proceedings of the 1996 DPF/DPB Summer Study on High-Energy Physics (Stanford Linear Accelerator Center, Menlo Park, CA, 1997)].

The inductance of the vacuum chamber of the Proton Storage Ring at Los Alamos National Laboratory was intentionally increased by the introduction of ferrite rings to counteract the longitudinal space-charge effect of the intense beam. The magnetic permeability of the ferrite could be adjusted by introducing current into solenoids wound around the ferrite. Results show that the minimum rf voltage necessary to stabilize the beam against e-p instability may be reduced over that previously measured. The injected bunch length was observed to be longer when the ferrite was heavily biased so that its effect was reduced.